1. The Field of the Invention
The invention generally relates to lasers used in sensor applications. More specifically, the invention relates to lasers used in applications facilitating changes in laser characteristics as light is reflected back into the laser.
2. Description of the Related Art
Lasers are becoming more widely used in many applications. Lasers may be used as a light generating source in fiber-optic communication applications. Laser light may also be used in various sensor applications, such as for example, self mixing applications.
Self mixing applications make use of laser power reflected back into the laser cavity and more especially into the active region. This reflected laser power changes the output of the laser. The output of the laser may be changed such as by changing the wavelength of the laser or by causing the laser output to be modulated in various beat patterns. A change in the wavelength of the laser, however, may be almost undetectably small. Thus the change in the wavelength may be detected as a shift in phase of an optical signal.
By continuously monitoring output from the laser, information can be gathered about conditions external to the laser. For example, using appropriate modulation and digital signal processing, information can be gathered about the distance of objects from a laser, movement of objects about the laser and the like. This allows the laser to be used in applications such as imaging, linear measurements, cursor pointers and the like. Methods of using lasers in self mixing applications are described in more detail in U.S. Pat. No. 6,707,027 titled “Method Of Measuring The Movement Of An Input Device” issued to Liess, et al. on Mar. 16, 2004, which is; incorporated herein by reference.
Self mixing applications make use of a laser's sensitivities to temperature and reflected light to achieve the self mixing functionality. For example, by varying the temperature of the laser, the frequency of the laser light that is output by the laser can be varied. Thus, temperature variations can be used to create chirp, i.e., a controlled increase and decrease in frequency, which can be used in a Doppler Effect range and motion sensor. When portions of a chirped signal are reflected back into the laser active region, the laser changes its output characteristics. These output characteristics can be detected to determine various actions that are occurring outside the laser as described above.
Unfortunately, lasers used in self mixing applications are typically manufactured with characteristics that counteract some of the sensitivities to reflected light or that counteract attempts to modulate the temperature, and hence the frequency, of the laser. For example, lasers are generally constructed to eliminate or reduce light reentering the laser cavity. Further, lasers are generally constructed to operate at a given temperature and to dissipate heat quickly so as to avoid changes in the laser wavelength.